It is already known that, for example sulphuric acid can be concentrated in apparatus consisting of an oven through which one or more long quartz tubes are located vertically. The acid is passed vertically downwards through the tube or tubes which are heated externally by means of hot stack gases generated in the oven and brought into direct contact with the tube or tubes so that heat is effectively transferred to the downward flowing acid. With regard to this method of heating it is consequently of great importance that constant heating of and conduction in the tube or tubes be achieved in order to avoid the occurrence of hot patches or sections of the tube or tubes during the concentration process as tensile strains and breaks or cracks in the tube or tubes can thereby be caused.